Latches made entirely out of cast iron appeared in the 1840s
and remained popular into the 20th century. Thomas [?] Blake took
some of the earliest patents for these devices, so they have come
to be known as Blake latches, although other manufactures also
made them.

Victorian-ear latches, because they were cast, often had low-relief
ornament over every available surface. The outline shapes of the
backplates, grasps, etc., took on lively curvilinear forms. Earlier
latches would have up to seven separate pieces to be ssemnled
and attached; "Black" latches came pre-assembled in
three pieces:

Numerous dated examples found, show that Blake's typical cast-iron
thumb-latch, with circular catch-plate mortised and screwed into
the door lintel, hollow patent bar-pivot, hollow staple guard,
and saucer lift with opposite down-curve, patented by United States
patent No. 1704, July 21, 1840, first came into general use on
and after that year.

It seems probable that this latch was preceded by rare cast-iron
experiments or improvements, i.e. cast-iron grasps on older wrought
latches of the Fig. 12 type, etc., and was closely followed by
evasive copies or patent infringements. But Blake's latch was,
and still is [1923] the cast-iron latch par excellence,
and without concerning ourselves with earlier un-patented predecessors
or variations of it or copies or patent infringements of its very
typical catch or staple, this latch, when complete and original,
as the evidence clearly shows, will date a house built after 1840.

Although an earlier patent for a cast-iron latch
was granted to the same patentees (Philo Blake and John Blake,
nephews of Eli Whitney) in 1833, this is the form which provied
the most popular and which infouenced much cast-iron latch making
thereafter. Blake worked out of New Haven, Connecticut.
From: Streeter, Donald. "The Historical Development of
Hand Firged Iron Builder's Hardware". In Ward, Jandl H.,
editor. The Technology of Historic American Buildings.
Wastington, DC: Association for Preservation Technology, 1983.